IPv4 Multicast Route Table

Beginning with Release 12.1, all IP multicast routes are stored and maintained in the software multicast route table. Routes are added to the multicast route table from the following sources:

Multicast static routes (configured manually by the network administrator)
Multicast dynamic routes (learned through protocols such as MBGP and MISIS)

The multicast route table is used for reverse path forwarding (RPF) checks, not for packet forwarding. The switch uses RPF checks to avoid multicast forwarding loops. When a multicast packet is received, the switch does an RPF lookup, which checks the routing tables to see if the packet arrived on the interface on which the router would send a packet to the source. The switch forwards only those packets that arrive on the interface specified for the source in the routing tables.

The RPF lookup uses the multicast routing table first, and if no entry is found for the source IP address, the RPF lookup uses the unicast routing table.

Note

Because the multicast routing table is used only for RPF checks (and not for routing), IP route compression and ECMP do not apply to multicast routes in the multicast routing table.

Beginning with ExtremeXOS software version 12.1, the route metric is no longer used to select between multicast and unicast routes. If the RPF lookup finds a route in the multicast table, that route is used. The unicast routing table is used only when no route is found in the multicast table.

The advantage to having separate routing tables for unicast and multicast traffic is that the two types of traffic can be separated, using different paths through the network.